A cable television distribution network with the above features is known from SPIE Vol. 1817 Optical Communications (1992), pp. 12-22, particularly from FIGS. 3 and 6. There, the fiber-optic distribution network is a ring network. The subscriber-assigned video signals, which have to be transmitted from the center (referred to there as "basic unit") to the subscribers of the cable television distribution network, are transmitted as analog signals. The video signals are modulated onto subcarriers of different frequencies, and the modulated subcarriers (37 subcarriers) are combined into a frequency-division-multiplexed signal which is transmitted to optical network terminations (referred to there as "optical network units", ONUs) together with the cable television signals using wavelength-division multiplexing. Table 1 shows that several wavelengths must be used to enable a total of 400 cable television subscribers to access individual video programs. This shows that to implement the video-on-demand service in cable television distribution networks of, e.g., more than 4000 subscribers, a great number of wavelengths is necessary. This involves a great amount of complexity in the wavelength-division demultiplexers in the optical network terminations and, thus, high costs. Another disadvantage of the prior art system is that in each optical network termination, for each subscriber connected thereto, a selection of a subset of programs is made from the entirety of the programs transmitted to the optical network termination. This requires, however, that the electrical access network extending from an optical network termination to a group of subscribers connected thereto should be a star network, as shown in FIG. 6 of the cited publication. This limitation, however, is not compatible with the structure of cable television distribution networks currently under discussion, such as the cable television distribution network described in "Elektrisches Nachrichtenwesen", 4. Quartal 1992, pp. 58-65, particularly FIG. 3, since for such networks, a tree structure rather than a star structure of the electrical access network is typical.
From JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 10, NO. 11, November 1992, pp. 1760-1765, a cable television distribution network is known which is also designed to provide video-on-demand service. There, the video signals (and the associated audio signals) are transmitted not as analog signals, but as digital signals which are the result of a compressing analog-to-digital conversion. As the multiplexing technique for combining the digital video signals, frequency-division multiplexing is preferred over time-division multiplexing, since the suitable time-division-multiplexing methods are claimed to be either not flexible enough or too expensive in the foreseeable future. The optical transmission uses the same optical carrier with which the cable television signals are distributed. All video signals are transmitted to all subscribers, e.g., 200-400 subscribers. However, for the cable television distribution networks currently under discussion, in which the fiber-optic network does not extend to the subscribers, but rather an electrical access network is provided between optical network terminations of the fiber-optic distribution network and the subscribers, this is not possible, because the electrical access network has a much lower transmission capacity than the fiber-optic distribution network.